When the nominal operation of a receive antenna of a satellite in orbit is tested, the radiation pattern of the receive antenna is tested and compared to expected operating specifications.
The invention is applied in particular for the in orbit testing of a telecommunication satellite but also any satellite the payload of which is made up of an uplink receive antenna, a downlink transmit antenna, and at least one transparent transponder (bent pipe transponder) connected between the uplink receive antenna and the downlink transmit antenna or the payload of which is made up of an uplink receive antenna, a transmit antenna and at least one regenerative transponder having at least one power telemetry measurement on its uplink transmission subsystem representing the input power of the transponder.
The known methods of testing the payload of a satellite in orbit are most often based on the use of a test signal on an unmodulated carrier, i.e. a sinusoidal signal, sometimes called a pure carrier. This test signal is generated, amplified and transmitted on the uplink via a ground station having a transmit ground antenna. The payload of the satellite receives the unmodulated test signal via an uplink receive antenna, the signal is propagated through the transponder and retransmitted to the ground station via a downlink transmit antenna. From measurements carried out on the downlink signal when the transponder operates in linear and transparent mode it is possible to characterize the response of the receive antenna of the satellite.
It is also known to employ unmodulated test signals in the form of multi-carrier test signals to test the operation of a multibeam receive antenna or a multi-frequency receive antenna, i.e. to generate at the same time a plurality of sinusoidal pure carriers distributed across a band of frequencies.
The known test methods based on the use of unmodulated test signals and described above have many disadvantages.
A first technical problem is raised by the limitation of the testing of the receive antenna of the satellite to a portion of the coverage area of the transmit antenna. In fact, for the test method to be used, the test ground station, which at the same time transmits the test signal on the uplink and acquires the signal retransmitted transparently by the satellite on the downlink, must be positioned in the area of intersection of the coverage areas of the receive antenna and the transmit antenna of the satellite. It is therefore not possible to test the receive antenna throughout its angular coverage.
A second technical problem is linked to the use of test signals intended to test the operation of a multibeam receive antenna or a multi-frequency receive antenna. This necessitates the generation of multi-carrier test signals and a device for generating those signals that increases the complexity of the test system.
Finally, and generally, when one or more unmodulated test carriers are transmitted by the test ground station, a third technical problem is caused by the existence of interference created by the ground station with other adjacent and operational satellites, which interference is harmful and unacceptable for those adjacent satellites in service and necessitate global frequency coordination and consequently specific arrangements for the IOT measurements.
In fact, the high power spectral density of one or more unmodulated carriers, which can be as high as approximately 70 dB above that of a modulated carrier, leads to severe frequency coordination constraints.
Of the specific approaches to global frequency coordination, a first approach consists in choosing an in orbit test (IOT) longitude of the satellite under test different from the final service orbital position so that the satellite under test does not interfere with adjacent satellites in service. This approach is sometimes suitable and in particular suits a geostationary satellite.
A second approach to the IOT measurements consists in choosing test time periods during the night to limit the effects of interference on the adjacent satellites, the traffic of which may be reduced during these nocturnal periods.
A third arrangement consists in performing the IOT measurements using test frequencies offset relative to the service frequencies of the satellite under test and that lie within the guard bands of the adjacent satellites.
However, such arrangements are costly and take a long time to implement, as well as limiting the IOT measurements that it is required to carry out in terms of the ranges of the parameters that it is required to characterize, the number of configurations of the payload tested, and the duration of the measurements, and can even prevent the carrying out of some IOT measurements.